Conventionally, spiral electrode groups are known in which a positive electrode plate and a negative electrode plate are wound with a separator interposed therebetween. As disclosed in JP-A-2001-319678, there is a spiral electrode group in which a separator for reinforcement is provided on a radially outer side of a winding-start end of the positive electrode plate. The separator on the radially outer side of the winding-start end of the positive electrode plate is reinforced to prevent a short circuit caused by a corner portion, a burr of the winding-start end of the positive electrode plate, or the like.
Some consumer-use nickel metal hydride batteries or the like are configured such that a negative electrode plate is arranged not only in a radially outer circumference of a positive electrode plate but in an inner circumference of a winding-start end in order to achieve high energy density by utilizing a charge-discharge capacity of a positive electrode to the maximum. An energy storage device having the above-described configuration has a problem that a short circuit is caused by a corner portion, a burr or the like not only on the radially outer side of the winding-start end of the positive electrode plate but also on the radially inner side.
Thus, as shown in FIG. 6, the configuration is such that one separator for reinforcement (sub-separator) is provided in a central portion of a separator in an expanded state (refer to a lower drawing of FIG. 6), and that the sub-separator is located both on the radially inner side of the winding-start end of the positive electrode plate and on the radially outer side of the winding-start end in a state wound spirally (refer to an upper drawing of FIG. 6).